This application claims a priority based on Japanese Patent Application No.6-165915 (165915/94) filed Jun. 24, 1994. Diamond has excellent physical and chemical properties. Current technology, however, cannot make a large diamond crystal yet. The ultra-high pressure synthesis method can produce bulk diamonds which are at large several millimeters in diameter. The diamond granules are too small to make devices thereby. There is no wide diamond crystal which has a wide area and can be safely called a wafer yet. Unfortunately, the Bridgman method, the Czochralski method or any other method cannot pull a thick diamond single crystal up from a melt unlike Si or GaAs. There is no effective method of growing a large diamond single crystal yet. The difficult of making a bulk crystal prevents from producing a bulk diamond wafer.
The use of a material in the electronics devices requires at least one inch diameter of a wafer. Furthermore 2 inch, 3 inch or 5 inch diameter wafers are desired for fabricating electronic devices on them. The applicability to the production line of devices demands less than 1 mm thickness of a wafer. Thus, the application of diamond to the device technology ardently requires a polycrystalline diamond wafer having a diameter of more than 1 inch and a thickness of less than 1 mm.
Further, a single crystal diamond wafer would be more convenient for making electronic devices of high quality. The feasible production of high quality single crystal wafers of Si or GaAs allowed the manufactures to make lots of Si or GaAs devices with stable performance.
Films of diamond can now be produced by a vapor phase synthesis method. The chemical vapor phase method produces a film of diamond on a pertinent substrate by heating the substrate in a reaction chamber, supplying a material gas to the reaction chamber, exciting the material gas, inducing a chemical reaction and depositing the result of the chemical reaction on the substrate. The material gas includes at least hydrogen gas and a hydrocarbon gas. In addition, boron-including gas, or nitrogen-including gas or other impurity gas is also supplied into the reaction chamber for changing the electric property or other properties by the impurity doping.
The chemical vapor deposition method is classified by the ways of exciting the material gas into a heat filament method, a microwave plasma CVD method, a radio-frequency plasma method, a DC plasma CVD method and so forth. Some method enables to make a wide diamond film. The speed of synthesis, however, is not fast. Thus, it is rather difficult to produce a thick diamond film by the method.
Nevertheless, there is not yet such a material as can be called a "diamond wafer" in the true meaning. The present technology Is still incapable of making a polycrystalline diamond wafer. Furthermore, nobody has succeeded in producing a single crystal diamond wafer.
Since no big bulk diamond can be produced yet, it is entirely impossible to make a homogeneous diamond wafer which consists only of diamond. A complex wafer may be produced even by the current technology by depositing a diamond film on a pertinent substrate. Since the diamond film is so thin that the substrate should remain for reinforcing the strength of the film. Thus, the complex wafer would consist of a diamond film and a substrate. If such a complex wafer of diamond were produced, the diamond film would be still polycrystalline. Thus the current technology is distanced far away from a single crystal diamond wafer. The single crystal diamond would be the most useful for all utilities. Since no big diamond bulk single crystal cannot be made at present, it is entirely impossible to make a monocrystalline diamond by homoepitaxy.
Some methods of heteroepitaxy were proposed by Japanese Patent Laying Open No.63-224225 (224225/1988), Japanese Patent laying Open No.2-233591 (233591/1990) and Japanese Patent Laying Open No.4-132687 (132687/1992).
They alleged that diamond single crystal films were heteroepitaxially grown on a SiC substrate, a Si substrate, a nickel substrate or a cobalt substrate.
Electronic devices or abrasion-resistant discs often make use only of a surface of a material. Thus, such a complex wafer may be useful to some extent. There is, however, no complex diamond wafer which is truly applicable for industrial purposes.